For Cristiano Ronaldo who needs to score additional 94 goals to reach the 1000 mark, I think he can do it if he continues to stay fit like he's been in recent years but for Lionel Messi, I don't see him reaching 1000 goals. Lionel Messi who is currently playing for Major League Soccer club Inter Miami, getting such amount of goal I think is almost impossible but for Cristiano Ronaldo, his current scoring streak is more likely to help him achieve that feat.
Just like you rightly said, age is no longer on their sides which I think is the major factor that might stop them reaching that milestone.
In what league do they have to play in order to score valid goals for the total counter? Well, if there aren't any restrictions, I think it's quite possible for them to hit the 1000 goals milestone yet. They just have to play at lower leagues where their age won't be an obstacle for their performance. They could even score a dozen goals in a single match.
After certain age, there isn't any lower division that would matter because they would not be able to play. Remember, most of their life these players played with other great players as well, sure they were always the best player but they didn't had terrible players around them, they had some great players around them most of their life. Sure right now maybe they do not have the greatest players, but they still have "some" decent players with them as well, so they are benefiting that too.
But if they go to for example like 4th division of Spanish league, just to score goals, the hard tackle defense of other teams would hurt them a lot and would injure them, age would stop them, and their terrible teammates would not be able to pass them well enough. So after a certain age, there is no way they can keep scoring, not like they can be 50 and go to lower league and score. Unless they just need like couple goals, then they can do that for a few games, score a few goals and be done.